Phase and hierarchical structure polymorphism in wild boar tusk enamel

Wild boar tusk enamel is crucial for the survival of wild boars, as it enables the tusks to endure complex stress conditions far exceeding those of human tooth enamel. However, a comprehensive understanding of the structure-property relationship of wild boar tusk enamel remains lacking. Herein, we systematically reveal polymorphism in the phases (crystalline, nanotwin, and amorphous structures) and structures (parallel, deflected, and decussate-arranged nanowires) of tusk enamel across multiple scales, hierarchies, and regions. In situ mechanical tests demonstrate that these polymorphisms facilitate unique energy dissipation behavior, including nanotwin-mediated toughening, stress-induced amorphization, and nanoscale interfacial sliding. These characteristics impart the tusk enamel with exceptional mechanical properties, including high stiffness, strength, viscoelasticity, and fracture toughness. Our results offer deeper insight into the structure and mechanics of wild boar tusk enamel and serve as a blueprint for the development of engineering composites with enhanced energy dissipation and durability.